Sorry to hear about the damage to the preamp, Halcro.
Concerning the feedback question, a high gain feedback loop that is unstable at some frequency or frequencies does not require much if any input stimulus for oscillations to occur. The reason that the problem occurred mainly with MC cartridges is most likely that you were using a considerably higher amount of phono stage gain with them, and perhaps also differences in frequency or phase response in the phono stage between when the circuitry that provides added gain for LOMC's is switched in and switched out. The reason that the problem does not occur with the other turntable is most likely that the characteristics of how it transmits acoustic/mechanical feedback to the cartridge are simply different, as a function of frequency and otherwise.
So what is probably happening is that some extremely tiny (and inevitable) low frequency vibration causes the cartridge to output a tiny signal, which is amplified by a high gain factor, resulting in an output from the nearby speaker that causes further vibration to be mechanically transmitted to the cartridge, which is further amplified by that high gain factor, etc., etc. The root cause of the problem is that the overall loop is unstable at some low frequency or frequencies, when the overall gain through the loop is above a certain amount.
I therefore agree with the others that the turntable needs to be re-located.
Best regards,
-- Al
Concerning the feedback question, a high gain feedback loop that is unstable at some frequency or frequencies does not require much if any input stimulus for oscillations to occur. The reason that the problem occurred mainly with MC cartridges is most likely that you were using a considerably higher amount of phono stage gain with them, and perhaps also differences in frequency or phase response in the phono stage between when the circuitry that provides added gain for LOMC's is switched in and switched out. The reason that the problem does not occur with the other turntable is most likely that the characteristics of how it transmits acoustic/mechanical feedback to the cartridge are simply different, as a function of frequency and otherwise.
So what is probably happening is that some extremely tiny (and inevitable) low frequency vibration causes the cartridge to output a tiny signal, which is amplified by a high gain factor, resulting in an output from the nearby speaker that causes further vibration to be mechanically transmitted to the cartridge, which is further amplified by that high gain factor, etc., etc. The root cause of the problem is that the overall loop is unstable at some low frequency or frequencies, when the overall gain through the loop is above a certain amount.
I therefore agree with the others that the turntable needs to be re-located.
Best regards,
-- Al